1. Field of the Invention
This invention relates to a process for preparing acetic acid from methanol and carbon monoxide or a mixed gas of carbon monoxide with hydrogen.
2. Description of the Prior Art
The so-called synthesis gas obtained by partial combustion reaction, steam reforming, etc. of coal, petroleum, natural gas, etc. has been known in the art to be used as a mixed gas of high content hydrogen with carbon monoxide which is a starting material for the preparation of acetic acid, and usually contains hydrogen in an amount of 20 to 75% by volume.
For example, Japanese Patent Publication No. 43767/85 discloses an alcohol carbonylation catalyst formed by supporting nickel, cobalt, compounds thereof and the like on a carbon support, and further discloses a process for preparing acetic acid which comprises subjecting methanol and carbon monoxide to a vapor phase catalytic reaction in the presence of the above catalyst and an iodine compound promoter, but neither teaches nor suggests the use of a mixed gas of high content hydrogen with carbon monoxide, i.e. the so-called synthesis gas as a starting material, and neither teaches nor suggests the use of a carbon-supported rhodium catalyst, resulting in unsatisfactory selectivity and yield to acetic acid.
Chemistry Letters, pp. 895-898, 1987, Fujimoto et al. reports promotion effect of hydrogen on vapor phase carbonylation of methanol over nickel on active carbon catalyst, and further reports that the yield of acetic acid reaches the maximum level at the H.sub.2 /CO ratio of 0.1 or above, and shows that presence of hydrogen in such an amount as a H.sub.2 /CO ratio of 0.1 or above remarkably improves the yield of acetic acid compared with the use of a hydrogen-free carbon monoxide, but with unsatisfactory results.
Proceeding of 9th International Catalysis Congress, 3, pages 1051-1058 (1988), Fujimoto et al. teaches that activity of methanol carbonylation on Rh is the highest compared with other metals such as Ni, Pd, Co and the like, and that addition of hydrogen as in the reaction conditions of 523.degree. K., 11 atm, W/F=5 g.multidot.h/mol (Rh: 1 g.multidot.h/mol), and CO/MeOH/MeI/H.sub.2 =50/9/1/(19) molar ratio improves activity of carbonylation compared with the case where no hydrogen is added, showing unsatisfactory yields to acetic acid.
Japanese Patent Publication No. 3334/72 discloses a process for preparing acetic acid which comprises effecting methanol carbonylation in the liquid phase in the presence of a rhodium complex catalyst and a promoter comprising methyl iodide or hydrogen iodide to obtain acetic acid at high yields, but has such disadvantages that the reaction system is so corrosive that use of costly corrosion-resistant material is required, that by-production of methane is increased, and so forth, and neither teaches nor suggests the use of a mixed gas of high content hydrogen with carbon monoxide, i.e. the so-called synthesis gas as a starting material.
Japanese Patent Application Laid-Open No. 299248/89 discloses a process for the production from methanol of acetic acid which process comprises contacting methanol with a gaseous mixture of carbon monoxide and hydrogen in the presence of a catalyst comprising rhodium and nickel supported on a carbon support and methyl iodide promoter with unsatisfactory results such as low methanol conversion, low selectivity and yield to acetic acid, increase of the by-produced methane, and the like.
In the case of ammonia synthesis, for example, it has been known in the art that methanation of a mixed gas comprising carbon monoxide and hydrogen of 99% by volume or more may be performed at relatively low temperatures in the presence of a catalyst comprising nickel or the like.
However, methanation of a mixed gas comprising high content carbon monoxide and low content hydrogen by use of a conventionally used catalyst comprising nickel or the like results in raising such problems that decomposition of carbon monoxide results in deposition of carbon, reduction in yield of carbon monoxide, deterioration of the catalyst and in blocking in the reactor, that the use of nickel catalyst results in formation of nickel carbonyl followed by sublimation of the nickel carbonyl, and that strong adsorption of carbon monoxide onto the surface of the catalyst results in hindering the reaction.